1. Field of the Invention
The present invention relates to systems and methods for navigation and control of spacecraft, and in particular to a system and method for determining the attitude of a spacecraft having at least one star sensor.
2. Description of the Related Art
Satellites enjoy widespread use for a variety of applications, including communications, surveillance, and data gathering. To perform their design mission, most satellites require accurate information regarding the spacecraft and payload attitude.
For applications where high bandwidth satellite attitude data is required, such data is typically obtained by on-board inertial measurement instruments such as inertial reference units having a plurality of gyros and accelerometers. However, while such instruments can provide high bandwidth information regarding the spacecraft attitude, they can only do so with respect to a datum attitude reference. Errors in this attitude reference propagate throughout the satellite navigation system. At the same time, it occasionally occurs that satellites lose their attitude reference, and must reacquire the reference with on-board sensors such as sun and earth sensors. When the satellite's attitude datum has been entirely lost (a lost-in-space-scenario), the satellite must determine its attitude reference completely anew. When the accuracy of the attitude datum has been compromised, but not lost, (a confused-in-space scenario), the satellite must also determine its attitude reference anew.
Most attitude acquisition algorithms assume a "Lost-in-Space" problem, which use no a priori attitude-related knowledge, and use a single star tracker to estimate the satellite attitude. Such "Lost-in-Space" algorithms can take a substantial amount processing capacity and memory. Unfortunately, current attitude acquisition algorithms have no capability to relax memory or processing requirements when attitude-related information is supplied, or deal with multiple star trackers. For example, the system disclosed U.S. Pat. No. 5,745,869, issued to van Bezooijen on Apr. 28, 1998 is designed to work with data from a single star tracker, and assumes a lack of a priori satellite attitude information.
Further, many acquisition algorithms are designed to work with "smart" star trackers that are capable of using a built-in star catalog to determine which star is being tracked. Such an algorithm is disclosed in U.S. Pat. No. 5,412,574, issued to Bender, Parks, and Brozenec on May 2, 1995. Such systems cannot utilize minimum capability star trackers, such as the CT-602 available from the BALL CORPORATION, which can only identify star positions (in the star tracker reference frame) and corresponding star instrument magnitudes.
Hence, there is a need for a device and method for estimating the attitude of a spacecraft using supplied a priori information and information from multiple star trackers. The present invention satisfies that need.